Microencapsulation is a process in which thin films or coatings or solid/gel matrix surround tiny particles or droplets that could be of any state of matter (solids, liquids or gases). Resultant sealed minicapsules are known as microcapsules or microspheres collectively termed as microparticles, which are typically spherical in shape and contain active material or core material surrounded by continuous wall or trapped in the solid or gel matrix. The active material that is encapsulated is known as the core, internal phase, fill, payload, or nucleus, whereas the material encapsulating is known as shell, coating, membrane, or wall material. Microcapsules may vary in size ranging from sub-micrometer to several millimeters. Typically, the average particle size is in the ranges from 5-300 micron in diameter. Most microcapsules have diameters between a few micrometers and a few millimeters.
The aim of microencapsulation is to provide a substance (active) in a finely divided state, to preserve it from degradation by limiting its exposure to the external environment (e.g. heat, moisture, acid, air, light) and to release it at a controlled rate under specific conditions on demand. It also helps the manufacturers cost effectiveness by having control of the optimal usage of the active. Further, by encapsulation volatile, sensitive, and reactive compounds can be turned into stable ingredients. Thus, microencapsulation itself not only is an added-value technique but also produces ingredients with numerous features. Moreover, masking the flavour of an active and providing uniform dispersion are also salient features of micro encapsulation.
Hydrocolloid micro-particles can be prepared, for example, by the emulsification method, or by the dripping method. One shortcoming of the emulsification method is that of residual organic solvent left in the microparticles, while the dripping method has been found to be unsuitable for scale-up and subsequent applications. Large-scale preparation methods for hydrocolloid particles, such as alginate particles, have been previously disclosed. However, residual organic solvent remaining in the microparticles is constantly a concern. An air-atomisation technique has been investigated to produce alginate-polylysine micro-capsules of Bacillus Calmette Guerin (BCG). This method is based on batch processing, of spraying sodium alginate into a calcium chloride bath, from which micro-gels are then separated.
A variety of techniques exist for microencapsulation of active ingredients. The techniques have associated benefits but also drawbacks.
For example, in spray drying material to be encapsulated is dissolved thoroughly with a carrier material (such as modified starch, maltodextrin, gum etc) and this solution is then fed into spray drier and atomised with a nozzle/spinning wheel. Hot air in the drier evaporates water and particles are collected at the bottom of the drier. Whilst an economical process, with this method there is a limited choice of coating material available, as mostly aqueous feed is used so that the resultant wall material is soluble in water. Heat can also cause degradation and/or oxidation of some active materials during drying and subsequent loss. Adherence of the core materials to the surface of the particles also increases the risk of oxidation. Spray dried capsules are water-soluble and therefore, the integrity of the capsules can be lost during rehydration.
Extrusion techniques can be used for volatile and unstable flavours in carbohydrate matrices. In this method carbohydrates, mixture of sucrose and maltodextrin are fed and melted by heat and sheared in an extruder so that crystalline structure is changed into amorphous form. Carbohydrates like sugar, starch hydrolysate are mostly used as encapsulating matrix. The encapsulant (active) is added through another opening at a cool side of the extruder near the extruder die. Core material dissolves in carbohydrate matrix and then forced through series of dies which enable its shape. The coating material hardens when it comes into contact with the liquid and causes encapsulation. This technique can provide excellent stability against oxidation and heat sensitive materials like Lactobacillus acidophilus can be encapsulated. However, extrusion methodology typically produces large particle size (500-1000 μm) which limits its sensory use. Moreover this method is expensive, sophisticated and complex to perform. Extruded granules may also show stickiness and clumping, and pay load concentrations can be low.
In co-crystallization, sucrose is used as a matrix for the coating and entrapment of a core material. Sucrose syrup is first concentrated to a super saturated state at high temperature (above 120° C.) and low moisture (95-97° Brix) to prevent crystallization. Active material is then added to the concentrated syrup with vigorous agiatation. As the temperature continuously increases, the syrup reaches a temperature where transformation and crystallization starts and heat is emitted. Encapsulated material is discharged and dried. In this approach the load of active material is generally very low and the process cannot be used for encapsulating heat sensitive materials.
Gel encapsulation involves the encapsulation of an active material in a gel (usually an alginate) matrix. The gel is formed by cross-linking of a polymer in which the core materials are suspended or dissolved. The microparticles can be formed by a variety of techniques including includes extrusion, emulsification, air atomization, electrostatic atomization, jet break-up, spinning disk atomization and by using a micro-nozzle array. However, none of these approaches is without practical difficulties as noted below.
Extrusion is a non-continuous process which tends to produce microparticles of only large size, and scale up can be problematic.
With the emulsification process, the gel particles produced have oil attached and removal of oil, or separation of oil from the aqueous phase of the gel particles, is required. This can be messy, difficult and time-consuming. The process is also not continuous.
With air atomization a coating material (e.g. sodium alginate) is extruded through a syringe pump into an air atomizer device and sprayed into a reagent bath (e.g. calcium chloride). Divalent calcium ions cross link the sodium alginate droplets and form microgel particles. Scaling up can be a problem as large number of beads cannot be formed due to limited interfacial surface of the calcium bath. It is a non-continuous method and there is difficulty in separating the beads.
In electrostatic atomization (electrospray or electrohydrodynamic atomization (EHDA)) electrified liquid is dispersed to fine droplets where electrostatic force is working on the charged surface of a liquid. Coating material is supplied to a nozzle electrode by using a micro syringe pump and dc high voltage is applied to the nozzle against an earth electrode. The droplets are dropped down into aqueous calcium chloride. It is a non-continuous method and has less scale up potential. It also tends to be a complicated operation.
The jet break-up approach is further divided into two methods, i.e vibration nozzle technology and jet-cutter technology. With the vibration nozzle method a suspension solution is forced through a small orifice. The liquid jet breaks into equally sized droplets by a superimposed vibration and solidified to form particles. Beads are produced that are spherical in shape with diameter range of 0.1-3.0 mm. This is a non-continuous method used only for small scale production of beads as beads are formed one after the other.
With jet cutter technology a fluid suspension is passed at high velocity through a nozzle in the form of a jet. The nozzle jet is cut into small pieces by rotating cutting wires. These cut at regular intervals forming same sized drops which are gelled in a bath of crosslinking reagent. Though the production rate can be increased by up to five fold when compared with the vibrating nozzle method, a still higher production rate would be preferred.
Spinning disk atomization is mostly used in biotechnology and medical fields. This technology permits production capacities from grams/min when using a single disc up to tons/day for a multi disk system. In this method alginate solution is delivered on the disk via a peristaltic pump with variable flow rates. Alginate droplets of desired sizes are collected in a CaCl2 bath. This tends to be a non-continuous method.
Use of a micronozzle array involves feeding a coating solution (e.g. alginate) into a stream of oil (soybean oil) through a MN (micro-nozzle) array in an upper stream and a cross-linking agent (CaCl2) through the downstream area of the oil from another MN array. The alginate and CaCl2 droplets collide with one another in the oil and reaction takes place resulting in the form of gel particles. This tends to be a complex method and there is limited scale up potential.
With a Voretx-Bowl Disk Atomizer System coating material is fed onto a rotating disk through a distributor present at the center above the disk. A vortex bowl is attached to the top of the shaft. Gelation solution (CaCl2) is fed to the centre of the bowl underneath the disk. All disks are rotating. Centrifugal force creates a wall along the inner side of the bowl. Alginate ejecting through the disk collides the wall and gel particles are formed and collected from a vat of CaCl2. This process can be difficult to operate however.
The present invention seeks to provide a microgel encapsulation technique that does not suffer the drawbacks associated with the various techniques. Thus, the present invention seeks to provide a process that can be operated in a continuous manner and that is cheap and easy to operate. The present invention also seeks to provide a process that can produce a small size microparticle (for example of less than 50 μm) and that lends itself to scale up for microparticle production.